FUEL CELL SYSTEM

PROBLEM TO BE SOLVED: To provide a fuel cell system in which the internal resistance of a fuel cell can be measured reliably regardless of the load on the fuel cell.SOLUTION: The fuel cell system comprises a DC-DC converter 3 which applies an AC signal of a predetermined frequency to the output signal from a fuel cell 1, a current measurement device 51 which detects the output current from the fuel cell 1, a voltage sensor 52 which detects the output voltage from the fuel cell 1, internal resistance calculation means 53 for calculating the internal resistance based on the output current and the output voltage when an AC signal is applied by the DC-DC converter 3, and amplitude voltage determination means 600 for determining the amplitude voltage of the AC signal being applied by the DC-DC converter 3. The amplitude voltage determination means 600 determines the amplitude voltage of the AC signal being applied by the DC-DC converter 3 to increase as the slope of the current/voltage characteristics indicating the relation of the output current and the output voltage increases.